a. Field of the Invention
The present invention relates to an FM multiplex stereo demodulating circuit, and more particularly to a demodulating circuit utilizing a sub-carrier generating circuit which comprises a so-called phase locked loop.
B. Description of the Prior Art
In the conventional FM multiplex stereo demodulating circuit, a pair of low-pass filters having a cut-off frequency of 15 kHz are provided in order to eliminate the pilot signal of 19 kHz, which is contained in the input of the demodulator of the switching circuit and is unnecessary for the audio output signal, from said input of the demodulator by sufficiently attenuating the frequency components of the 19 kHz pilot signal. To this end, each one of the pair of low-pass filters must have a steep cut-off characteristic, and such a low-pass filter has to be provided in each of the left and the right channels of the stereophonic signal channels. However, in the conventional FM multiplex stereo demodulating circuit as mentioned above, accuracy of electrical values of the component parts is severely required since the more the cut-off characteristic of the low-pass filter is made steep, the more easily the filter characteristic exhibits ripples in the audio frequency band. Moreover, the central frequency f.sub.0 of each low-pass filter must be precisely adjusted. Instead of a pair of the low-pass filters, it is possible to use a band-rejection filter at 19 kHz which is to be connected to the input terminal of the multiplex stereo demodulating circuit. However, this system is of no practical use since the separation characteristic will considerably deteriorate, though it has the advantage that only one filter is enough for the elimination of the 19 kHz pilot signal.